Asphalt binder (which can also be referred to as asphalt, bitumen or asphalt cement) is a petroleum residue available in many varieties depending upon its natural origins and on the industrial process used in its production. Chemically, asphalt binders are typically a mixture of aliphatic, aromatic and naphthenic hydrocarbons with high molecular weight and small quantities of organic acids, bases and heterocyclic components containing nitrogen and sulfur. Asphalt is a colloidal substance in which a dispersed phase comprising asphaltene is covered by a protective phase of polar resins in micelles which are dispersed in a phase comprising oils. The chemical nature of the various phases is not readily definable. Generally, however, the nucleus has characteristics that are more aromatic than naphthenic; the protective resins are prevalently naphthenic and the oils, which cover the micelles, have a paraffinic character. The properties of asphalt can be associated with the balance of the percentages of its components. Due to the difficulty of performing an exact chemical analysis, a classification is normally accepted which is based upon fractionated precipitation of the bitumen using selective solvents and an elution of the solubles in a chromatographic column (American Society for Testing and Materials (ASTM) Standard S2007-75). Identification of an asphalt or bitumen is made by combining the results of this analysis with the values of penetration, softening and penetration index. Physically, bitumen is a visco-elastic material, with viscous flow under slow stress and at high temperatures and more elastic behavior under rapid stress at low temperature.
Due to its wide availability, relatively low price, and ease of application, asphalt has found widespread use as a road-building material, notwithstanding its visco-elastic behavior. Intrinsic limitations can accompany the use of asphalt as a road-building material. Asphalt can demonstrate softening and unwanted flow at high temperatures, brittleness and unwanted fracturing at low temperatures, poor mechanical and elastic characteristics and a tendency to aging with exposure. Mineral aggregate is frequently added to asphalt (to provide “asphalt concrete”) to modify its rheology and temperature susceptibility. Roads are frequently laid with a base course and binder layers that insulate the upper asphalt surface from the ground. The upper asphalt road surface can develop extremely hot temperatures during the summer months and extremely cold temperatures during the winter months. The rheology of asphalt is such that, notwithstanding mineral additives, at high temperatures, it will flow in response to stresses imposed by vehicular traffic and develop “ruts” that not only provide unacceptable surface for vehicular travel, but provide localized areas of unacceptable thickness which crack under loads imposed by vehicular traffic at cold temperatures during winter and form pits (often referred to as “chuckholes”).
Efforts have been made to improve asphalt performance by adding various asphalt modifiers, including various types of polymers, as well as carbonaceous materials, such as carbon black and carbon fiber. For instance, carbon fibers have been added to asphalt in hopes that the high tensile strength of carbon fiber can increase the cracking resistance of asphalt pavement. See Abtahi, S. M., et al., Construction and Building Materials, 2010, 24(6): 871-877. Asphalt mixtures modified with meso-length carbon fibers show resistance to permanent deformation, high tensile strength at low temperatures and high fatigue resistance. See Cleven, M. A., “Investigation of the Properties of Carbon Fiber Modified Asphalt Mixtures.” M. S. Thesis, Department of Chemical Engineering, Michigan Technological University, 2000; and Jahromi, S. G., and A. Khodai, The Arabian Journal for Science and Engineering, 2008, 33(2B): 355-364. However, it is believed that addition of meso-fibers could produce fiber clumps, leading to poor dispersion of fibers and non-uniform mixing. See Cleven, M. A., “Investigation of the Properties of Carbon Fiber Modified Asphalt Mixtures.” M. S. Thesis, Department of Chemical Engineering, Michigan Technological University, 2000. Nano-sized fibers can also be used, and are expected to behave more like nanoparticles, allowing for more uniform mixing.
Carbon black has shown reinforcing effect on rubbers and has potential as an additive to asphalt binder due to their common carbon-based nature. See Chaala, A., et al., Fuel, 1996, 75(13): 1575-1583. Pelletized carbon black can reduce the temperature susceptibility of asphalt, improve rutting resistance at high temperatures, and reduce stripping potential. See Chaala, A., et al., Fuel, 1996, 75(13): 1575-1583; Rostler, F. S., et al., Association for Asphalt Paving Technologists Proc., 1977, 46: 376-410; Khosla, N. P., Transportation Research Record: Journal of the Transportation Research Board, 1991, 1317: 10-22; Yao, Z., and L. C. Monismith, Association of Asphalt Paving Technologists, 1986, 55: 564-585; and Park, T., and C. W. Lovell, “Using Pyrolyzed Carbon Black from Waste Tires in Asphalt Pavement (Part 1, Limestone Aggregate);”Publication FHWA/IN/JHRP-95/10; Joint Highway Research Project, Indiana Department of Transportation and Purdue University, West Lafayette, Ind., 1996. Yet, fatigue and cracking resistance of asphalt mixtures modified by carbon black can remain a concern.
Accordingly, there is an on-going need for new asphalt modifiers for use in asphalt (i.e., in asphalt binder) and in asphalt mixtures (e.g., in asphalt concrete) that can improve performance. For example, there is a need for new modifiers that can reduce temperature susceptibility and/or increase rutting resistance, moisture resistance and/or cracking resistance of hot mix asphalts.